Friction Stir Welding (FSW) is a conventionally known method of joining a plurality of steel members together. Friction stir welding is a method in which: a protrusion provided at the tip of a welding tool is, while being rotated, pressed against a welding target portion of a plurality of steel members to be joined, so that the welding target portion and its surrounding portion are softened due to frictional heat and the welding target portion and its surrounding portion, which have been softened, are stirred and thereby a plastic flow is caused; then the welding tool is removed from the welding target portion and the welding target portion is cooled down; and as a result, the plurality of steel members are joined together.
Patent Literatures 1 and 2 disclose an apparatus and a method of performing spot joining using the above-described friction stir welding. Patent Literature 1 discloses a friction stir apparatus configured to reduce wear of a protrusion of a welding tool by softening a welding target portion with laser light before inserting the welding tool into the welding target portion. Patent Literature 2 discloses a friction stir welding method, in which the operation of a welding tool is controlled in accordance with the temperature of a welding target portion of a plurality of steel members on which welding is being performed, and thereby the welding target portion is suppressed from transforming into a martensitic structure when cooling down. In the welding method, a receiving member is disposed at the opposite side to the welding tool, with the welding target portion positioned between them, and the receiving member receives pressure from the welding tool. The receiving member includes a temperature sensor configured to measure the temperature of the welding target portion. When the temperature of the welding target portion, which is detected by the temperature sensor, reaches or exceeds an A3 transformation point, processing conditions such as the rotational speed and the pressing force of the welding tool are adjusted. The A3 transformation point is a temperature at which a metallographic structure is transformed into an austenite structure. After the processing conditions are adjusted, when the temperature of the welding target portion is reduced to an A1 transformation point or lower, the welding tool is removed from the welding target portion. The A1 transformation point is a eutectoid temperature.